1. Field of the Invention
The present invention relates to a high resistivity silicon carbide single crystal from which substrates for the processing of high frequency semiconductor devices such as MESFETs can be made. The composition of the high resistivity substrates is chosen so that trapping effects deteriorating the performances of the devices are eliminated. The invention also relates to silicon carbide substrates which have a high resistivity before being processed and are able to maintain a high resistivity after processing steps involving heating at temperatures above 1500° C.
2. Background and Prior Art
Semi-insulating, or high resistivity, silicon carbide (SiC) substrates are required for the fabrication of SiC and III-N power microwave devices with low RF losses. SiC devices with power densities exceeding those of GaAs and Si LDMOS technologies have been realized, however, there are still a number of issues that need to be addressed in order to realize the full potential of SiC based microwave technologies. For example, the density of electrically active defects present in the semi-insulating substrate can influence the characteristics of high-frequency SiC devices by inducing device parasitic trapping effects, also known as backgating effects. Semi-insulating SiC substrates can also have a higher micropipe density than conductive substrates and tend to suffer lower crystal growth yields.
In the prior art, essentially two methods are disclosed for the manufacturing of high resistivity (ρ≧105 Ω·cm) SiC crystals. In the first method, described by U.S. Pat. No. 5,611,955, deep levels are introduced during the crystal growth by doping silicon carbide with a heavy or transition metal such as vanadium, or a combination of a transition metal and an electrically passivating impurity such as hydrogen, chlorine and fluorine. In the disclosed invention, the passivating impurities are proposed to neutralize the concentration of shallow donors and acceptors.
In the second method, described by U.S. Pat. No. 6,218,680 B1 (hereafter referred to as '680), intrinsic point defects are introduced during the silicon carbide growth to compensate the dominating type of shallow acceptor or donor dopants, whereas the concentration of heavy metals or transition elements dopants is kept as low as possible, in such a way that they do not affect the electrical properties of the silicon carbide crystal. For example, '680 cites a concentration of 5×1016 cm−3 for the shallow nitrogen dopant and a concentration below 1014 cm−3, or below the detection limit of analytical measurements, for the vanadium transition metal.
It is now quite well established that producing semi-insulating silicon carbide crystal by vanadium doping can in certain practical situations lead to detrimental side effects such as a deterioration of the crystal quality and low process yields. For example, if the vanadium concentration incorporated into the crystal is too high and exceeds the solubility limit of vanadium in SiC (3–5×1017 cm−3, see Jenny J. R. et al., Appl. Phys. Lett 68(14), p. 1963 (1996)), additional micropipes defects and vanadium rich precipitates are created (see Balakrishna V. et al., Mat. Res. Soc. Symp. 572, p. 245 (1999) and Bickermann M. et al., J. Cryst. Growth 233, p. 211 (2001)).
The complications related to a high concentration doping of transitions metals such as vanadium are overcome in patent '680 which introduces the use of points defects, or “intrinsic defects”, as deep levels to compensate the free carriers introduced in the crystal by the shallow donors or acceptors.
There are a number of native point defects, such as vacancies, antisites and interstitials, that could be present in a crystal. In the silicon carbide compound semiconductor, both a silicon vacancy (VSi), the absence of a Si atom at a Si-site of the perfect crystal lattice, and similarly a carbon vacancy (VC) can occur. Two antisite defects, the silicon antisite (SiC) resulting from the incorporation of a Si atom at a C-site of the crystal lattice and the carbon antisite (CSi) are possible. Similarly, two interstitials, ISi and IC, can result from the misplacement of either a silicon or a carbon atom from a lattice site to a position in between two or more perfect lattice sites. Pairs, complexes and precipitates of these intrinsic defects may also be formed. It is however to date not practically clear by which method the formation of a specific type of intrinsic defect can be either enhanced or suppressed during the crystal growth process itself.
However, certain intrinsic defects can be thermally unstable and if used in a semi-insulating silicon carbide substrate, the resistivity of the substrate may not be well controlled under certain conditions. For example, it has been reported that the silicon vacancy can be annealed out when the silicon carbide crystal is subjected to a sufficiently long high temperature treatment.
In Schottky gate field effect transistors (MESFETs) processed on semi-insulating silicon carbide substrates prepared from crystals grown with a different composition than the preferred embodiments of the disclosed invention, it has been observed that traps present in these substrates can cause a collapse of the MESFET drain-source current. This current collapse is for example visible after application of a high drain-source voltage and can be reversed either by applying heat or a light source to the device, so that the trapped carriers are released (FIG. 15a). This effect deteriorates the device characteristics, as the device is unstable and has a power handling capability lower than what it is designed for. Drain current collapse has also been encountered in GaN MESFETs and AlGaN/GaN HEMTs, and undesired steady-state and transient phenomena have more extensively been studied in GaAs RF devices. In particular, traps present in semi-insulating GaAs substrates, such as the EL2 antisite deep-donor or the Cr deep-acceptor, have been shown to affect the characteristics of compound devices.